Stickler enraged and in bad mood (6)
I believe the answer is:
pedant
'stickler' is the definition.
(I've seen this before)
'enraged and in bad mood' is the wordplay.
'enraged' is an anagram indicator.
'in' indicates putting letters inside.
'bad mood' becomes 'pet' (informal term for sulking).
'and' with letters rearranged gives 'dan'.
'dan' inserted into 'pet' is 'PEDANT'.
(Other definitions for pedant that I've seen before include "Panted like dogmatist" , "Know-all who insists on strict observance of rules" , "One is overly-concerned with detail" , "Panted (anag.)" , "Old teacher" .)